The cross-linking of a thermoplastic is a preferred way of improving its resistance to plastic flow over a wider temperature range and, consequently, for obtaining better dimensional stability of shaped objects.
Several processes have been proposed for cross-linking thermoplastics such as polyvinyl chloride and polyolefins. These processes are based on radical-like reactions, condensation reactions, or nucleophilic substitution reactions.
Therefore to bring about the cross-linking of polyethylene the SIOPLAST process of Dow Chemical proposes the radical-like grafting of vinyl monomers containing alkoxysilane groups which lead, by hydrolysis, to silanol groups which by polycondensation bring about the cross-linking of polyethylene. This process is used in the industrial field in cable manufacturing. However, it has the disadvantage of causing a cross-linking density gradient because this is controlled by the diffusion of water in the finished object, water which is necessary for the hydrolysis reaction of the alkoxysilane functions.
For cross-linking polyvinyl chloride, there have been developed processes following a photochemical route, an electronic bombardment and by gamma radiation by subjecting the polymer to these radiations in the presence of a multifunctional monomer such as for example tetraethylene glycol dimethacrylate or trimethylol propane trimethacrylate, the cross-linking reaction being provided by grafting and polymerization of these monomers on the polymer skeleton.
The major disadvantages of these processes are the sensitivity of polyvinyl chloride viz-a-viz these radiations which bring about its degradation by elimination of HCl in a cascade process, and also the presence of a large quantity of monomer to obtain effective and homogeneous cross-linking which has the consequence of modifying its intrinsic properties. The processes by gamma radiation and electronic bombardment require moreover a high level of technology and expensive apparatus which limits their wide-spread use.
To overcome these disadvantages processes have been proposed for polyvinyl chloride which require a nucleophilic substitution reaction of the chlorine atoms by alkali or alkaline-earth thiolates or by mixed thiolate- alkaline or alkaline-earth carboxylate compounds. These processes are well adapted for bringing about cross-linking on a coating line, but they have the major disadvantage of strongly sensitizing the cross-linked material to photochemical degradation induced by traces of disulphides which are always present and are difficult to eliminate in the presence of thiol compounds. For this reason the development of these processes has not become wide-spread.
The European Patent No. 0,032,587 proposes a cross-linking process for acrylic ester copolymers by diols, or diesters of diols, in the presence of a titanate of general formula Ti(OR)4 wherein, the four OR groups can be identical or different. The presence of titanate in the cross-linked material can lead under the action of humidity to the formation of TiO2, a well-know catalyst of photo-oxidation reactions. This can therefore be a major disadvantage to the use of such materials outside.
The British Patent No. 907,775 describes a process for treating polyvinyl acetate with a metal alcoholate of formula M(OR)n, wherein M can be in particular titanium or tin, the reaction is carried out at 80.degree. C. in a solvent and leads to a substitution of the acetate radicals by titanate or stannate radicals which leads to the formation of an organometal polymer.
Therefore a cross-linking process for thermoplastic polymers or copolymers is still being sought which is simple, inexpensive, results in materials which are not sensitive to photochemical aging and which can be integrated into forming operations on an industrial production line such as extrusion, calandering, coating, injection and thermoforming.